The invention under this application is aimed to enable substantial reduction of compression power required for air separation, that is, liquefaction and rectification of air to separate oxygen, nitrogen and other materials and in particular to extract them as liquid products.
Most of the costs to separate air into and extract oxygen, nitrogen and other materials are that of power, and most of this power is consumed to compress feed air. Therefore reduction of this compression power is immediately contributive to the amount of power per unit volume of the products. Various solutions have been proposed along this line e.g. utilization of the cold of LNG (liquefied natural gas), based on the fact that power to compress gas is reduced by lowering the inlet temperature of gas. However in a plant to produce, say 10,000 m.sup.3 /h of oxygen, where feed air of five times as much as the product is required, the accrued saving is counterbalanced either (1) by a larger capital and power costs of adsorbing facilities to remove moisture, carbon dioxide and other impurities, which is necessary to avoid solidification in the process or (2) in a generally accepted method of cooling feed air and removing such impurities by the use of a regenerative cooler or a reversing heat exchanger, by a required level of feed air compression so as to enable removal of impurities. That is, removal of impurities by a regenerative cooler or a reversing heat exchanger requires in general the pressure of feed air to be 5 kg/cm.sup.2 G. This means that despite the use of LNG the power saving is not largely expected, and the contribution of the cold of LNG is limited to supplement cold energy in liquefaction/rectification stages.